Fire curves

Types of fire exposure

In recent years a great deal of research has taken place internationally to ascertain the types of fire which could occur in tunnel and underground spaces. This research has taken place in both real, disused tunnels and laboratory conditions.
As a consequence of the data obtained from these tests, a series of time/temperature curves for the various exposures have been developed as detailed.

Cellulosic curve

Standard fire tests to which specimens of constructions subject to are based on the use of the Cellulosic time/temperature curve, as defined in various national standards, e.g. ISO 834, BS 476 : part 20, DIN 4102, AS 1530 etc. Although there are other types of fire test curve e.g. BS 7436, the curve as detailed below for this exposure is the lowest used in normal practise. This curve is based on the burning rate of the materials found in general building materials and contents.

The temperature development of the Cellulosic fire curve (ISO-834) is described by the following equation: T = 20+345*LOG(8*t+1).

Hydrocarbon

Although the Cellulosic curve has been in use for many years, it soon became apparent that the burning rates for certain materials e.g. petrol gas, chemicals etc, were well in excess of the rate at which for instance, timber would burn. As such, there was a need for an alternative exposure for the purpose of carrying out tests on structures and materials used within the petrochemical industry, and thus the hydrocarbon curve was developed.

The hydrocarbon curve is applicable where small petroleum fires might occur, i.e. car fuel tanks, petrol or oil tankers, certain chemical tankers etc. In fact, although the hydrocarbon curve is based on a standardised type fire, there are numerous types of fire associated with petrochemical fuels.

The temperature development of the Hydrocarbon (HC) fire curve is described by the following equation:
T = 20+1080*(1-0,325*e-0,167*t-0,675*e-2,5*t)

Hydrocarbon Modified curve

Derived from the above-mentioned Hydrocarbon curve, the French regulation asks for an increased version of that Hydrocarbon curve, the so called HydroCarbon Modified curve (HCM).

The maximum temperature of the HCM curve is 1300ºC instead of the 1100ºC, standard HC curve.
However, the temperature gradient in the first few minutes of the HCM fire is as severe as all Hydrocarbon based fires (RWS, HCM, HC), possibly causing a temperature shock to the surrounding concrete structure and concrete spalling as a result of it.

The temperature development of the Hydrocarbon Modified (HCM) fire curve is described by the following equation:
T = 20+1280*(1-0,325*e-0,167*t-0,675*e-2,5*t)

RABT ZTV curve

The RABT curve was developed in Germany as a result of a series of test programmes such as the Eureka project. In the RABT curve, the temperature rise is very rapid up to 1200°C within 5 minutes. The duration of the 1200°C exposure is shorter than other curves with the temperature drop off starting to occur at 30 minutes for car fires. The drop off for train fires only starts at 60 minutes. The 110 minutes cooling period is applied to both fire curves.

The failure criteria for specimens exposed to the RABT-ZTV time/temperature curve is that the temperature of the reinforcement should not exceed 300°C. There is no requirement for a maximum interface temperature.

The temperature development of the RABT-ZTV fire curve(s) is described by the following co-ordinates:

RABT-ZTV (train)

Time (minutes)

Temperature (°C)

0

15

5

1200

60

1200

170

15

RABT-ZTV (car)

Time (minutes)

Temperature (°C)

0

15

5

1200

30

1200

140

15

RWS (Rijkswaterstaat) curve

The temperature development of the RWS fire curve is described by the following co-ordinates:

The RWS curve was developed by the Rijkswaterstaat, Ministry of Transport in the Netherlands. This curve is based on the assumption that in a worst case scenario, a 50 m³ fuel, oil or petrol tanker fire with a fire load of 300MW could occur, lasting up to 120 minutes. The RWS curve was based on the results of testing carried out by TNO in the Netherlands in 1979.

The correctness of the RWS fire curve as a design fire curve for road tunnels was reconfirmed in the Full Scale Tests in the Runehamar tunnel in Norway.

The temperature development of the RWS fire curve is described by the following co-ordinates:

RWS, RijksWaterStaat

Time (minutes)

Temperature (°C)

0

20

3

890

5

1140

10

1200

30

1300

60

1350

90

1300

120

1200

180

1200

The difference between the RWS and the Hydrocarbon curve, is that the latter is based on the temperatures that would be expected from a fire occurring within a relatively open space, where some dissipation of the heat would occur. The RWS curve is based on the sort of temperature you would find when a fire occurs in an enclosed area, such as a tunnel, where there is little or no chance of heat dissipating into the surrounding atmosphere.

The RWS curve simulates the initial rapid growth of a fire using a petroleum tanker as the source, and the gradual drop in temperatures to be expected as the fuel load is burnt off.

The failure criteria for specimens exposed to the RWS time/temperature curve is that the temperature of the interface between the concrete and the fire protective lining should not exceed 380°C and the temperature on the reinforcement should not exceed 250°C.

Apart from the Netherlands, the following countries have adopted the RWS standard, either as their local legislation or have specified it for specific projects:

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